Terms use in Timber Design

Board Foot
- the content of a volume 12x12x1 inch, 144 cu. in. of ½ cu. ft.

Check
- separation along the grain, the greater part of which occurs across the annual rings. It generally
arises from the process of seasoning. Like shakes, checks also reduce the resistance to shear.

Dead Load
-applied to the weight of the materials of construction; the weight of beam, girders, flooring,
partitions, and so on.

Decay
-is the disintegration of wood substance due to the action of wood-destroying fungi. It is easily
recognized, for the wood becomes soft, spongy or crumbly. The growth of fungi is encouraged by air,
moisture and favorable temperature.

Elastic limit
-or sometimes called proportional limit. It is the unit stress that occurs after which the
deformation begins to increase to a faster rate than the increments of the applied load.

Elasticity
-is the property of a material that enables it to return to its original size and shape when the load
To which it has been subjected is removed.

Glued Built-up Member

-are structural elements, the section of which are composed of built-up lumber, wood structural
panels or wood structural panels in combination with lumber, all parts bonded together with adhesive.

Grade in Lumber
-necessary to identify the quality of lumber. Structural grades are established in relation to
strength of properties and use classification so that allowable stresses for the design can be assigned.
Individual grades of the various species are given a commercial designation such as No. 1 and No. 2.,
Select structural, and Dense No. 2 by the grading rules agency concerned.

Knot
-is a portion of a branch or limb that has been surrounded by subsequent growth of the tree

Live Load
-represents the probable load due to occupancy of a building and includes the weight of human
occupants, furniture, equipment, stored materials and snow.

Modulus of Elasticity
-measure the stiffness of the material.

M. G. COSTELO
Moisture Content of Wood
-defined as the ratio of the weight of water in a specimen to the weight of the oven-dry wood,
expressed as a percentage.

Nominal size in lumber

-designation for the individual piece of structural lumber by its cross-sectional dimensions. The
size is indicated by the breadth and depth of the cross section in inches.

Permanent Set
-or permanent deformation. This is beyond the elastic limit.

Seasoning of wood
-the process of removing moisture from the green wood. It is accomplished by exposing lumber
to the air for an extended period or by heating it in kilns.

Shake
-is a separation along the grain, principally between the annual rings. It reduces the resistance to
shear, and consequently members subjected to bending are directly by their presence. The strength of
members in longitudinal compression 9column, posts, etc.,) is not greatly affected by shakes.

Shearing Stress
-results from the tendency of two equal and parallel forces, acting in opposite directions, to cause
adjoining surfaces of a member to slide one on the other.

Working Strength Design

- (Alternative Method Design) a margin of safety is provided by permitting calculated flexural
stress to reach only certain percentage of the ultimate strength of the concrete or yielded strength of the
reinforcing.
Timber
- wood in any of stages from feeling through readiness for use of structural materials.

Common factors that influence the strength of wood.

a). DENSITY
-the difference in arrangement and size of the cell activities and the thickness of the cell
walls determine the specific gravity of various species of wood. The strength of wood is closely
related to its density.

b). NATURAL DEFECTS

-any irregularity in wood that effects its strength or durability is called a defect.

c). MOISTURE CONTENT

- Wood moisture content (WMC) is often used as an indicator of decay problems in

houses. This document describes the meaning and use of readings from a wood moisture meter.
M. G. COSTELO
Common defects on wood.
Defects may be naturally occurring or can be man-made. Natural defects can be due to many
reasons such as environmental factors, growth patterns, soil composition, etc. Man-made defects can
occur at many points from the felling of the tree, transport, storage, sawing, drying, etc.
Although you can work around some defects such as knots, or cut off defects such as splits,
boards that are heavily twisted, bowed, cupped, or crooked usually are not usable.

Bow A curve along the face of a board

that usually runs from end to end.

Checking A crack in the wood structure of a

piece, usually running lengthwise.
Checks are usually restricted to the
end of a board and do not penetrate
as far as the opposite side of a
piece of sawn timber.

Crook Warping along the edge from one

end to the other. This is most
common in wood that was cut from
the centre of the tree near the pith.

Cupping Warping along the face of a board

across the width of the board. This
defect is most common of plain-
sawn lumber.

Split A longitudinal separation of the

fibres which extends to the
opposite face of a piece of sawn
timber.

Twist Warping in lumber where the ends

twist in opposite directions.

M. G. COSTELO
Wane The presence of bark or absence of
wood on corners of a piece of
lumber.

Blue Stain A discoloration that penetrates the

wood fibre. It can be any colour
other than the natural colour of the
piece in which it is found. It is
classed as light, medium or heavy
and is generally blue or brown.

Machine A darkening of the wood due to

Burn overheating by the machine knives
or rolls when pieces are stopped in
a machine.

Pitch An accumulation of resinous

material on the surface or in
pockets below the surface of wood.
Also called gum or sap.

Loose Knot A knot that cannot be relied upon

to remain in place in the piece.
Caused by a dead branch that was
not fully integrated into the tree
before it was cut down.

Tight Knot A knot fixed by growth or position

in the wood structure so that it
firmly retains its place in the
surrounding wood.

Wormholes Small holes in the wood caused by

insects and beetles.

M. G. COSTELO
Philippine wood species
The wood species mentioned here, are the most commonly used in our own production. It
should be noticed that wood is UNIQUE, and there are no two pieces of wood that are exactly the
same, so when dealing with natural products it is important to remember that there can be variations,
and especially there are differences in appearance between flats awn and quarter sawn wood in
appearance, strengths and stability. Also there are differences even within a small region, and even
within a single tree.
Wood is a wonderful material, and it is self-sustaining when grown in well managed forests.

PHILIPPINE ROSEWOOD
Scientific Name: Petersianthus Quadrialatus
Weight: About 650 Kgs/m3
Color: Very Dark With Lighter Flames Naturally Occurring
Description: Philippine Rosewood is a very beautiful dark and flamy wood.
It has for many years been used for local boat making due
to its strength and durability. We have introduced this species
for interiors and flooring.

TEAK

Scientific Name: Tectona Grandis

Weight: About 600kgs.
Color: Brown
Description: Teak is one of the world’s best timbers. It’s usage is multiple,
but mainly furniture, decking, and various kitchen accessories.
Especially well suited for outdoor use.

PHILIPPINE MAHOGANY

Scientific Name: Shorea Negrosensis

Weight: About 500kgs/m3
Description: Also known as Lauan, the Philippine Mahogany is considered
the best in Asia. The types growing in LUZON are generally
harder and darker, while MINDANAO origin is a lighter and
milder type.

M. G. COSTELO
YAKAL

Scientific Name: Shorea Laevis

Weight: About 700 Kgs/m3
Color: Yellow To Golden Red
Description: Yakal is a hard and golden Mahogany type which is used for
frequently used products and surfaces. Ideal for outdoor use.

ALMON - Red Mahogany

Scientific Name: Shorea Almon

Weight: 450-500kgs/m3
Color: Uniform Light Red
Description: Almon grows in the southern island of Mindanao. It is uniform
in colour and weight, and is mild and easy to work.

BAGRAS - Southern Mahogany

Scientific Name: Eucalyptus Deglupta

Weight: 400-600kgs At 15%
Color: Reddish / Brown
Description: Also known as Mindanao Gum or Rainbow Eucalyptus. Both
natural and plantation growth. Furniture and cabinet making.

BAGTICAN - WHITE LAUAN

Scientific Name: Shorea And Parashorea

Weight: About 400-600kgs/m3
Color Pale to Light Red
Description: Light red or white Lauan and Bagtikan species are widespread
in the Philippines, but vary in weight from north to south.
Often used for joinery.

IGEM

Scientific Name: Podocarpus Imbricatus

Weight: 450-600kgs
Color: Light Yellow To White
Description: Igem is mainly used as a Ramin replacement for moldings and
frames.

M. G. COSTELO
MAHOGANY - (Plantation)

Scientific Name: Swietenica Macrophylla

Weight: About 500kgs/m3 At 12% M.c.
Color: Redbrownish With Orange Tone
Description: Swietenia Mahogany has been planted in the Philippines since
the 70ies. Originating in Brazil, where it is now an endangered
species, this plantation species can now be aquired on sustainable
basis. It may contain some small firm knots, but is available in
good quality for furniture purposes.

ACACIA (road side)

Scientific Name: Acacia Auriculaeformi, Racosperma Aurculiforme

Weight: About. 4-500kgs/m3
Color: Dark Brown, With Very Distingt Sap Wood (yellow)
Description: The Acasia grows wild everywhere in the Philippines, and is
often used for local handicrafts, and especially suited for turning
into bowls and plates.

ACACIA MANGIUM

Scientific Name: Acacia Mangium Willd. Leguminosae (mimosoideae)

Weight: 545kgs At 12%
Color: Pale Brown With Very Light Sapwood
Description: The sapwood of magnum is narrow and pale yellow to light
brown, while the heartwood is olive brown to gray brown, with
darker streaks. It is hard, with a medium texture, strong and
durable (not in contact with the ground). The grain shows an
interlocked figure radically, but looks straight on the flat sawn
surface.

NARRA

Scientific Name: Pterocarpus Indicus

Weight: About 600 Kgs/m3
Color: Deep Orange Golden To Darker Red Tones
Description: Narra is considered the most valuable wood in the Philippines,
and is therefore very restricted. Special permits are required for
export of finished products. - Most often used for furniture,
flooring, and panels.

M. G. COSTELO
PILI

Scientific Name: Canarium Luzonicum

Weight: About 500kgs/m3
Color: Whitish, Light Brown
Description: Fruit tree with a nut fruit, also sometimes called olive. Found
mainly in the Philippines. Common in primary forests and low
and medium altitudes. Common names: Antang, kedondong,
piling-liitan, belis, malapili.

COCONUT WOOD

Scientific Name: Cocos Nucifera

Weight: About 600kgs/m3
Color: Brown
Description: Coconut is very widespread all over the Philippines, and it
used extensively in the local construction industry. It is a very
hard wood which is excellent for turning into small bowls,
jewelry accessories, but also used for cutting boards, flooring
and much more.

GMELINA (White Teak)

Scientific Name: Gmelina Aborea

Weight: About. 400 Kgs At 12% M.c.
Color: Pale, Light Color
Description: One of the most used plantation species in the Philippines.
Widely used for fingerjointed and edgeglued materials for
shelves, furniture parts, and moldings.

OAK

Scientific Name: Quercus Rob.

Weight: About 600kgs/m3
Color: Pale/light
Description: Our Oak primarily comes from Northern Europe and is
lighter in color, and more dense than it’s North American
equivalent. Lead time for production in Oak will most often
be about 6 months, until production is stable, after which 3
months production time is normal.

BEECH WOOD
Scientific Name: Fagus Grandifolia
Color: Pale White.
Description: Mostly closed, straight grain; fine, uniform texture. Our Beech
comes from Northern and Central Europe

M. G. COSTELO
CHERRY

Scientific Name: Prunus, Serotina

Color: Golden Light Brown
Description:North American Cherry is one of Americas favorite cabinet and
furniture woods, prized for its rich reddish color and fine graining.

MAPLE

Scientific Name: Acer Saccharum

Color: Creamy White To Light Reddish Brown
Description: American white Maple is widely used for furniture, and is often
used for very pale products with a soft sanded surface.

SANTOL

Scientific Name: Sandoricum Koetjape

Weight: About 500 Kgs/m3
Color: Light Brown
Description: heartwood is pale reddish-brown when dry, imparting the color
to water. It is fairly hard, moderately heavy, close-grained and
polishes well. It is plentiful, easy to saw and work, and
accordingly popular. If carefully seasoned, it can be employed
for house-posts, interior construction, light-framing, barrels,
cabinetwork, boats, carts, sandals, butcher’s blocks, household
utensils and carvings. When burned, the wood emits an aromatic
scent.

MOUNTAIN PINE

Scientific Name: Pinus

Color: Light Reddish
Weight: About 350-400kgs/m3
Description: Pine is grown above 1000 meters in the Philippines. It is
relatively fast grown, but we are able to offer most of our
products free of knots.

M. G. COSTELO
Types of wood

Wood is divided, according to its botanical origin, into two kinds: Softwoods from coniferous
trees and hardwoods from broadleaved trees. Structurally softwoods are generally simple in structure
and lighter whereas hardwoods are generally complex in structure and harder.

a). Softwood (like pine wood)

is much lighter and easier to process than the heavy hardwood. The density of softwoods ranges
between 350-700 kg/m³, the ability to process and dry softwood much more easily and faster
makes it the main supply of commercial wood today.

b). Hardwood (like fruit tree wood)

The density of hardwoods are 450-1250 kg/m³. Both consist of approximately 12 %
moisture (Desch and Dinwoodie, 1996). Due to the more dense and complex structure of
hardwood, the permeability is very low in comparison to softwood, thus making it more difficult
to dry.

Wood-water relationships

The timber of living trees and freshly felled logs contains a large amount of water, which often
constitutes more weight than the actual wood. Water has a significant influence on wood: wood
continually exchanges moisture (water) with its surroundings, although the rate of exchange is strongly
affected by the degree wood is sealed.

Water in wood may be present in two forms:

1. Free water: The bulk of water contained in the cell lumina is only held by capillary forces: it is
not bound chemically and is termed free water. Free water is not in the same thermodynamic
state as liquid water: energy is required to overcome the capillary forces. Furthermore, free water
may contain chemicals, altering the drying characteristics.
2. Bound or hygroscopic water: Bound water is bound to the wood via hydrogen bonds. The
attraction of wood for water arises from the presence of free hydroxyl (OH) groups in the
cellulose, hemicelluloses and lignin molecules in the cell wall. The hydroxyl groups are
negatively charged electrically. Water is a polar liquid. The free hydroxyl groups in cellulose
attract and hold water by hydrogen bonding.

Water in cell lumina may be in the form of water vapour, but the total amount is normally
negligible, at normal temperatures and moisture contents.

a). Moisture Content of Wood

The moisture content of wood is calculated by the formula (Siau, 1984). Here, is the
green mass of the wood, is its oven-dry mass (the attainment of constant mass generally after
drying in an oven set at 103 +/- 2 °C for 24 hours as mentioned by Walker et al., 1993). This can
also be expressed as a fraction of the mass of the water and the mass of the oven-dry wood rather
than a percentage, for example, 0.59 kg/kg (oven dry basis) expresses the same moisture content
as 59% (oven dry basis).
M. G. COSTELO
b). Fibre Saturation Point
When green wood dries, the first water to go is the free water from the cell lumina. It is
held only by the capillary forces. Most physical properties, such as strength and shrinkage, are
unaffected by the removal of free water. The fibre saturation point (FSP) is defined as the
moisture content at which free water should be completely gone, while the cell walls are
saturated with bound water. In most woods, the fibre saturation point is at 25 to 30% moisture
content. Siau (1984) reported that the fibre saturation point (kg/kg) is dependent on the
temperature T (°C) according to the following equation:

Many important properties of wood show a considerable change as the wood is dried
below the fibre saturation point. These include:

1. Volume: ideally no shrinkage occurs until some bound water is lost, i.e. until the wood is
dried below FSP.

2. Most strength properties show a consistent increase as the wood is dried below the FSP
(Desch and Dinwoodie, 1996). An exception is impact bending strength and, in some cases
toughness.

3. Electrical resistivity increases very rapidly with the loss of bound water when the wood
dries below the FSP.

c.) Equilibrium Moisture Content

Wood is a hygroscopic substance. It has the ability to take in or give off moisture in the
form of vapor. The water contained in wood exerts a vapor pressure of its own, which is
determined by the maximum size of the capillaries filled with water at any time. If the water
vapor pressure in the ambient space is lower than the vapor pressure within wood, desorption
takes place. The largest sized capillaries, which are full of water at the time, empty first. The
vapor pressure within the wood falls as water is successively contained in smaller and smaller
sized capillaries. A stage is eventually reached when the vapor pressure within the wood equals
the vapor pressure in the ambient space above the wood, and further desorption ceases. The
amount of moisture that remains in the wood at this stage is in equilibrium with the water vapor
pressure in the ambient space, and is termed the equilibrium moisture content or EMC (Siau,
1984). Because of its hygroscopic, wood tends to reach a moisture content that is in equilibrium
with the relative humidity and temperature of the surrounding air. The EMC of wood varies with
the ambient relative humidity (a function of temperature) significantly, to a lesser degree with
the temperature. Siau (1984) reported that the EMC also varies very slightly with species,
mechanical stress, drying history of wood, density, extractives content and the direction of
sorption in which the moisture change takes place (i.e. adsorption or desorption).

d). Moisture Content of Wood in Service

Wood retains its hygroscopic characteristics after it is put into use. It is then subjected to
fluctuating humidity, the dominant factor in determining its EMC. These fluctuations may be
more or less cyclical, such as diurnal changes or annual seasonal changes. In order to minimize
the changes in wood moisture content or the movement of wooden objects in service, wood is
usually dried to a moisture content that is close to the average EMC conditions to which it will
be exposed. These conditions vary for interior uses compared with exterior uses in a given
geographic location. For example, according to the Australian Standard for Timber Drying
M. G. COSTELO
 Quality (AS/NZS 4787, 2001), the EMC is recommended to be 10-12% for the majority of
Australian states, although extreme cases may be up to 15 to 18% for some places in
Queensland, Northern Territory, Western Australia and Tasmania. However, the EMC may be as
low as 6 to 7% in dry centrally heated houses and offices or in permanently air-conditioned
buildings.

The primary reason for drying wood to a moisture content equivalent to its mean EMC
under use conditions is to minimize the dimensional changes (or movement) in the final product.

e). Shrinkage and Swelling

Shrinkage and swelling may occur in wood when the moisture content is changed
(Stamm, 1964). Shrinkage occurs as moisture content decreases, while swelling takes place when
it increases. Volume change is not equal in all directions. The greatest dimensional change
occurs in a direction tangential to the growth rings. Shrinkage from the pith outwards, or radially,
is usually considerably less than tangential shrinkage, while longitudinal (along the grain)
shrinkage is so slight as to be usually neglected. The longitudinal shrinkage is 0.1 to 0.3%, in
contrast to transverse shrinkages, which is 2-10%. Tangential shrinkage is often about twice as
great as in the radial direction, although in some species it may be as much as five times as great.
The shrinkage is about 5 to 10% in the tangential direction and about 2 to 6% in the radial
direction (Walker et al., 1993).

Differential transverse shrinkage of wood is related to:

1. the alternation of late wood and early wood increments within the annual ring;
2. the influence of wood rays in the radial direction (Kollmann and Cote, 1968)
3. the features of the cell wall structure such as micro fibril angle modifications and pits; and,
4. the chemical composition of the middle lamella.

Wood drying

Wood drying may be described as the art of ensuring that gross dimensional changes through
shrinkage are confined to the drying process. Ideally, wood is dried to that equilibrium moisture content
as will later (in service) be attained by the wood. Thus, further dimensional change will be kept to a
minimum.

It is probably impossible to completely eliminate movement in wood, but this may be

approximated by chemical modification. This is the treatment of wood with chemicals to replace the
hydroxyl groups with other hydrophobic functional groups of modifying agents (Stamm, 1964). Among
all the existing processes, wood modification with acetic anhydride has considerable promise due to the
high anti-shrink or anti-swell efficiency (ASE) attainable without damaging the wood properties.
However, acetylation of wood has been slow to be commercialised due to the cost, corrosion and the
entrapment of the acetic acid in wood. There is extensive literature relating to the chemical modification
of wood (Rowell, 1983, 1991; Kumar, 1994; Haque, 1997).

Drying timber is one approach for adding value to sawn products from the primary wood
processing industries. According to the Australian Forest and Wood Products Research and
Development Corporation (FWPRDC), green sawn hardwood. However, currently-used conventional

M. G. COSTELO
drying processes often result in significant quality problems from cracks, both externally and internally,
reducing the value of the product. As an example, in Queensland alone (Anon, 1997), assuming that
10% of the dried softwood is devalued by $200 per cubic meter because of drying defects, saw millers
are losing about $5 million per year in that State alone. Australia wide this could be $40 million per year
for softwood and an equal or higher amount for hardwood. Thus proper drying under controlled
conditions (prior to use) is of great importance in timber utilization in any country, where climatic
conditions vary considerably at different times of the year.

Drying, if carried out promptly after the felling of trees, also protects timber against primary
decay, fungal stain and attack by certain kinds of insects. Organisms, which cause decay and stain,
generally cannot thrive in timber with a moisture content below 20%. Several, though not all, insect
pests can live only in green timber. Dried wood is less susceptible to decay than green wood (above 20%
moisture content).

Apart from the above important advantages of drying timber, the following points are also
significant (Walker et al., 1993; Desch and Dinwoodie, 1996):

1. Dried timber is lighter, and hence the transportation and handling costs are reduced.
2. Dried timber is stronger than green timber in most strength properties.
3. Timbers for impregnation with preservatives have to be properly dried if proper penetration is to
be accomplished, particularly in the case of oil-type preservatives.
4. In the field of chemical modification of wood and wood products, the material should be dried to
a certain moisture content for the appropriate reactions to occur.
5. Dry wood works, machines, finishes and glues better than green timber. Paints and finishes last
longer on dry timber.
6. The electrical and thermal insulation properties of wood are improved by drying.

Prompt drying of wood immediately after felling therefore results in significant upgrading of,
and value adding to, the raw timber. Drying enables substantial long term economy in timber utilisation
by rationalizing the utilization of timber resources. The drying of wood is thus an area for research and
development, which concerns many researchers and timber companies around the world.

a). How wood dries: the mechanisms of moisture movement

Water in wood normally moves from zones of higher to zones of lower moisture content
(Walker et al., 1993). In simple terms, this means that drying starts from the outside and moves
towards the centre, and it also means that drying at the outside is also necessary to expel
moisture from the inner zones of the wood. Wood, after a period of time, attains a moisture
content in equilibrium with the surrounding air (the EMC, as mentioned earlier).

1). Mechanisms for moisture movement

Moisture passageways
The basic driving force for moisture movement is chemical potential.
However, it is not always straightforward to relate chemical potential in wood to
commonly observable variables, such as temperature and moisture content (Keey
et al., 2000). Moisture in wood moves within the wood as liquid or vapour
through several types of passageways depending on the nature of the driving
force, (e.g. pressure or moisture gradient), and variations in wood structure
(Langrish and Walker, 1993), as explained in the next section on driving forces
M. G. COSTELO
 for moisture movement. These pathways consist of cavities of the vessels, fibres,
ray cells, pit chambers and their pit membrane openings, intercellular spaces and
transitory cell wall passageways. Movement of water takes place in these
passageways in any direction, longitudinally in the cells, as well as laterally from
cell to cell until it reaches the lateral drying surfaces of the wood. The higher
longitudinal permeability of sapwood of hardwood is generally caused by the
presence of vessels. The lateral permeability and transverse flow is often very low
in hardwoods. The vessels in hardwoods are sometimes blocked by the presence
of tyloses and/or by secreting gums and resins in some other species, as
mentioned earlier. The presence of gum veins, the formation of which is often a
result of natural protective response of trees to injury, is commonly observed on
the surface of sawn boards of most eucalypts. Despite the generally higher
volume fraction of rays in hardwoods (typically 15% of wood volume), the rays
are not particularly effective in radial flow, nor are the pits on the radial surfaces
of fibres effective in tangential flow (Langrish and Walker, 1993).

Moisture movement space

The available space for air and moisture in wood depends on the density
and porosity of wood. Porosity is the volume fraction of void space in a solid. The
porosity is reported to be 1.2 to 4.6% of dry volume of wood cell wall (Siau,
1984). On the other hand, permeability is a measure of the ease with which fluids
are transported through a porous solid under the influence of some driving forces,
e.g. capillary pressure gradient or moisture gradient. It is clear that solids must be
porous to be permeable, but it does not necessarily follow that all porous bodies
are permeable. Permeability can only exist if the void spaces are interconnected
by openings. For example, a hardwood may be permeable because there is
intervessel pitting with openings in the membranes (Keey et al., 2000). If these
membranes are occluded or encrusted, or if the pits are aspirated, the wood
assumes a closed-cell structure and may be virtually impermeable. The density is
also important for impermeable hardwoods because more cell-wall material is
traversed per unit distance, which offers increased resistance to diffusion (Keey et
al., 2000). Hence lighter woods, in general, dry more rapidly than do the heavier
woods. The transport of fluids is often bulk flow (momentum transfer) for
permeable softwoods at high temperature while diffusion occurs for impermeable
hardwoods (Siau, 1984). These mechanisms are discussed below.
b). Driving forces for moisture movement
Three main driving forces used in different version of diffusion models are moisture
content, the partial pressure of water vapour, and the chemical potential (Skaar, 1988; Keey et
al., 2000). These are discussed here, including capillary action, which is a mechanism for free
water transport in permeable softwoods. Total pressure difference is the driving force during
wood vacuum drying.

1). Capillary action

Capillary forces determine the movements (or absence of movement) of free
water. It is due to both adhesion and cohesion. Adhesion is the attraction between water
to other substances and cohesion is the attraction of the molecules in water to each other.

M. G. COSTELO
As wood dries, evaporation of water from the surface sets up capillary forces that exert a
pull on the free water in the zones of wood beneath the surfaces. When there is no longer
any free water in the wood capillary forces are no longer of importance.

2). Moisture content differences

The chemical potential is explained here since it is the true driving force for the
transport of water in both liquid and vapour phases in wood (Siau, 1984). The Gibbs free
energy per mole of substance is usually expressed as the chemical potential (Skaar,
1933). The chemical potential of unsaturated air or wood below the fibre saturation point
influences the drying of wood. Equilibrium will occur at the equilibrium moisture content
(as defined earlier) of wood when the chemical potential of the wood becomes equal to
that of the surrounding air. The chemical potential of sorbed water is a function of wood
moisture content. Therefore, a gradient of wood moisture content (between surface and
centre), or more specifically of activity, is accompanied by a gradient of chemical
potential under isothermal conditions. Moisture will redistribute itself throughout the
wood until the chemical potential is uniform throughout, resulting in a zero potential
gradient at equilibrium (Skaar, 1988). The flux of moisture attempting to achieve the
equilibrium state is assumed to be proportional to the difference in chemical potential,
and inversely proportional to the path length over which the potential difference acts
(Keey et al., 2000).

The gradient in chemical potential is related to the moisture content gradient as

explained in above equations (Keey et al., 2000). The diffusion model using moisture
content gradient as a driving force was applied successfully by Wu (1989) and Doe et al.
(1994). Though the agreement between the moisture-content profiles predicted by the
diffusion model based on moisture-content gradients is better at lower moisture contents
than at higher ones, there is no evidence to suggest that there are significantly different
moisture-transport mechanisms operating at higher moisture contents for this timber.
Their observations are consistent with a transport process that is driven by the total
concentration of water. The diffusion model is used for this thesis based on this empirical
evidence that the moisture-content gradient is a driving force for drying this type of
impermeable timber.
Differences in moisture content between the surface and the centre (gradient, the
chemical potential difference between interface and bulk) move the bound water through
the small passageways in the cell wall by diffusion. In comparison with capillary
movement, diffusion is a slow process. Diffusion is the generally suggested mechanism
for the drying of impermeable hardwoods (Keey et al., 2000). Furthermore, moisture
migrates slowly due to the fact that extractives plug the small cell wall openings in the
heartwood. This is why sapwood generally dries faster than heartwood under the same
drying conditions.

3). Moisture movement directions for diffusion

It is reported that the ratio of the longitudinal to the transverse (radial and
tangential) diffusion rates for wood ranges from about 100 at a moisture content of 5% to
2 to 4 at a moisture content of 25% (Langrish and Walker, 1993). Radial diffusion is
somewhat faster than tangential diffusion. Although longitudinal diffusion is most rapid,
it is of practical importance only when short pieces are dried. Generally the timber boards
are much longer than in width or thickness. For example, a typical size of a green board
used for this research was 6 m long, 250 mm in width and 43 mm in thickness.
M. G. COSTELO
c). Reasons for splits and cracks during timber drying and their control
The chief difficulty experienced in the drying of timber is the tendency of its outer layers
to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the
fibre saturation point while the interior is still saturated, stresses (called drying stresses) are set
up because the shrinkage of the outer layers is restricted by the wet interior (Keey et al., 2000).
Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses
across the grain exceed the strength across the grain (fibre to fibre bonding).

The successful control of drying defects in a drying process consists in maintaining a

balance between the rate of evaporation of moisture from the surface and the rate of outward
movement of moisture from the interior of the wood. The way in which drying can be controlled
will now be explained.

d). Influence of temperature, relative humidity and rate of air circulation

The external drying conditions (temperature, relative humidity and air velocity) control
the external boundary conditions for drying, and hence the drying rate, as well as affecting the
rate of internal moisture movement. The drying rate is affected by external drying conditions
(Walker et al., 1993; Keey et al., 2000), as will now be described.

Temperature: If the relative humidity is kept constant, the higher the temperature, the
higher the drying rate. Temperature influences the drying rate by increasing the moisture holding
capacity of the air, as well as by accelerating the diffusion rate of moisture through the wood.
The actual temperature in a drying kiln is the dry-bulb temperature (usually denoted by Tg),
which is the temperature of a vapour-gas mixture determined by inserting a thermometer with a
dry bulb. On the other hand, the wet-bulb temperature (TW) is defined as the temperature
reached by a small amount of liquid evaporating in a large amount of an unsaturated air-vapour
mixture. The temperature sensing element of this thermometer is kept moist with a porous fabric
sleeve (cloth) usually put in a reservoir of clean water. A minimum air flow of 2 m/s is needed to
prevent a zone of stagnant damp air formation around the sleeve (Walker et al., 1993). Since air
passes over the wet sleeve, water is evaporated and cools the wet-bulb thermometer. The
difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression, is used to
determine the relative humidity from a standard hygrometric chart (Walker et al., 1993). A
higher difference between the dry-bulb and wet-bulb temperatures indicates a lower relative
humidity. For example, if the dry-bulb temperature is 100 °C and wet-bulb temperature 60 °C,
then the relative humidity is read as 17% from a hygrometric chart.

Relative humidity: The relative humidity of air is defined as the partial pressure of water
vapour divided by the saturated vapour pressure at the same temperature and total pressure (Siau,
1984). If the temperature is kept constant, lower relative humidities result in higher drying rates
due to the increased moisture gradient in wood, resulting from the reduction of the moisture
content in the surface layers when the relative humidity of air is reduced. The relative humidity
is usually expressed on a percentage basis. For drying, the other essential parameter related to
relative humidity is the absolute humidity, which is the mass of water vapour per unit mass of
dry air (kg of water per kg of dry air). The following equation can be used to calculate the
absolute humidity from the relative humidity (Strumillo and Kudra, 1986):

M. G. COSTELO
 Air circulation rate: Drying time and timber quality depend on the air velocity and its
uniform circulation. At a constant temperature and relative humidity, the highest possible drying
rate is obtained by rapid circulation of air across the surface of wood, giving rapid removal of
moisture evaporating from the wood. However, a higher drying rate is not always desirable,
particularly for impermeable hardwoods, because higher drying rates develop greater stresses
that may cause the timber to crack or distort. At very low fan speeds, less than 1 m s-1, the air
flow through the stack is often laminar flow, and the heat transfer between the timber surface and
the moving air stream is not particularly effective (Walker et al., 1993). The low effectiveness
(externally) of heat transfer is not necessarily a problem if internal moisture movement is the key
limitation to the movement of moisture, as it is for most hardwoods (Pordage and Langrish,
1999).

d). Classification of timbers for drying

The timbers are classified as follows according to their ease of drying and their proneness
to drying degrade:

Highly refractory woods: These woods are slow and difficult to dry if the final product is
to be free from defects, particularly cracks and splits. Examples are heavy structural timbers with
high density such as ironbark (Eucalyptus paniculata), blackbutt (E. pilularis), southern blue
gum (E. globulus) and brush box (Lophostemon cofertus). They require considerable protection
and care against rapid drying conditions for the best results (Bootle, 1994).

Moderately refractory woods: These timbers show a moderate tendency to crack and
split during seasoning. They can be seasoned free from defects with moderately rapid drying
conditions (i.e. a maximum dry-bulb temperature of 85 °C can be used). Examples are Sydney
blue gum (E. saligna) and other timbers of medium density (Bootle, 1994), which are potentially
suitable for furniture.

Non-refractory woods: These woods can be rapidly seasoned to be free from defects
even by applying high temperatures (dry-bulb temperatures of more than 100 °C) in industrial
kilns. If not dried rapidly, they may develop discolouration (blue stain) and mould on the surface.
Examples are softwoods and low density timbers such as Pinus radiata.

Methods of drying timber

Broadly, there are two methods by which timber can be dried: (i) natural drying or air drying and
(ii) artificial drying.

a). Air drying

Air drying is the drying of timber by exposing it to the air. The technique of air drying
consists mainly of making a stack of sawn timber (with the layers of boards separated by
stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely
depends on climatic conditions, and on the air movement (exposure to the wind). For successful
air drying, a continuous and uniform flow of air throughout the pile of the timber needs to be
arranged (Desch and Dinwoodie, 1996). The rate of loss of moisture can be controlled by coating
the planks with any substance that is relatively impermeable to moisture; ordinary mineral oil is
usually quite effective. Coating the ends of logs with oil or thick paint, improves their quality
upon drying. Wrapping planks or logs in materials which will allow some movement of
moisture, generally works very well provided the wood is first treated against fungal infection by
M. G. COSTELO
coating in petrol/gasoline or oil. Mineral oil will generally not soak in more than 1-2 mm below
the surface and is easily removed by planing when the timber is suitably dry.

b). Kiln drying

The process of kiln drying consists basically of introducing heat. This may be directly,
using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although
solar energy is also possible. In the process, deliberate control of temperature, relative humidity
and air circulation is provided to give conditions at various stages (moisture contents or times) of
drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers,
called wood drying kilns, which are fitted with equipment for manipulation and control of the
temperature and the relative humidity of the drying air and its circulation rate through the timber
stack (Walker et al., 1993; Desch and Dinwoodie, 1996).

Kiln drying provides a means of overcoming the limitations imposed by erratic weather
conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost
all commercial timbers of the world are dried in industrial kilns. A comparison of air drying,
conventional kiln and solar drying is given below:

1. Timber can be dried to any desired low moisture content by conventional or solar kiln
drying, but in air drying, moisture contents of less than 18% are difficult to attain for most
locations.
2. The drying times are considerably less in conventional kiln drying than in solar kiln
drying, followed by air drying.
1. This means that if capital outlay is involved, this capital is just sitting there for a longer
time when air drying is used. On the other hand, installing an industrial kiln, to say nothing of
maintenance and operation, is expensive.
2. In addition, wood that is being air dried takes up space, which could also cost money.
3. In air drying, there is little control over the drying elements, so drying degrade cannot be
controlled.
4. The temperatures employed in kiln drying typically kill all the fungi and insects in the
wood if a maximum dry-bulb temperature of above 60 °C is used for the drying schedule. This is
not guaranteed in air drying.
5. If air drying is done improperly (exposed to the sun), the rate of drying may be overly
rapid in the dry summer months, causing cracking and splitting, and too slow during the cold
winter months.
The significant advantages of conventional kiln drying include higher throughput and better
control of the final moisture content. Conventional kiln and solar drying both enable wood to be
dried to any moisture content regardless of weather conditions. For most large-scale drying
operations solar and conventional kiln drying are more efficient than air drying.

Kiln drying schedules

Satisfactory kiln drying can usually be accomplished by regulating the
temperature and humidity of the circulating air to suit the state of the timber at any given
time. This condition is achieved by applying kiln-drying schedules. The desired objective
of an appropriate schedule is to ensure drying timber at the fastest possible rate without
causing objectionable degrade. The following factors have a considerable bearing on the
schedules.

M. G. COSTELO
 1. The species; because of the variations in physical, mechanical and transport
properties between species.
2. The thickness of the timber; because the drying time is approximately inversely
related to thickness and, to some extent, is also influenced by the width of the timber.
3. Whether the timber boards are quarter-sawn, back-sawn or mixed-sawn; because
sawing pattern influences the distortion due to shrinkage anisotropy.
4. Permissible drying degrades; because aggressive drying schedules can cause
timber to crack and distort.
5. Intended use of timber; because the required appearance of the timber surface and
the target final moisture contents are different depending on the uses of timber.

Considering each of the factors, no one schedule is necessarily appropriate, even

for similar loads of the same species. This is why there is so much timber drying
research, including this work, focused on the development of effective drying schedules.

c). Drying defects

Drying defects are the most common form of degrade in timber, next to natural problems
such as knots (Desch and Dinwoodie, 1996). There are two broad categories of drying defects
(some defects involve both causes):

 Defects that arise due to the shrinkage anisotropy. This leads to warping: cupping,
bowing, twisting, spring and diamonding.
 defects that arise due to uneven drying. This leads to the rupture of the wood tissue:
checks (surface, end and internal), end splits, honey-combing and case hardening. Another such
defect is collapse, often seen as a corrugation, or “wash boarding” of the wood surface (Innes,
1996). Collapse is a defect that results from the physical flattening of fibbers, above the fiber
saturation point (thus not a form of shrinkage anisotropy).

Australian and New Zealand Standard Organizations (AS/NZS 4787, 2001) have
developed a standard for timber quality. Their five criteria for measuring drying quality:

1. moisture content gradient and presence of residual drying stress (case-hardening);

2. surface, internal and end checks;
3. collapse;
4. distortions;
5. and discoloration caused by drying.

M. G. COSTELO
 RULES IN DRESSING OF LUMBER

 for dimension ≤ 100mm deduct 5mm on each face.

 For dimension > 100mm deduct 6.25mm on each face.

Ex.

Dressed size= ?
150mm

100mm
Given: d= 150mm
B= 100mm

Sol’n.:
Since d > 100mm, and Since b ≤ 100mm,
d = 150 – 2(6.25) b = 100 – 2(5)
= 137.50mm = 90mm

M. G. COSTELO